There are a number of simple, portable, wireless telephones already available on the market in accordance with which a portable telephone coacts with a neighbouring stationary unit which is connected to a telephone line. One radio frequency is used for transmitting speech from the stationary to the portable unit, another radio frequency being used for transmission in the opposite direction; a call normally being transmitted in full duplex. From the calling aspect, the equipment is often entirely unselective, the receivers being opened so-called squelch circuit, but equipment is also known in which some kind of selective call is used, inter alia, for reducing the risk of interception by adjoining equipment. A description of existing equipment with a single-channel wireless telephone and selective call is to be found in an article by Tsujimura, Kuwabara: Cordless Telephone System and Its Propagation Characteristics, IEEE, Trans. on Vehicular Technology, Vol. VT-26, No. 4, New York Nov. 1977, pages 367-371.
One difficulty with known wireless telephones is mutual interference between adjacent telephones. An account is given in the article mentioned above of field strengths around wireless telephones, and it is stated that from the point of view of interference the same duplex channel for a wireless telephone with a range of 20 meters cannot be planned for a city center with tower blocks at a density greater than one per 600.times.600 m..sup.2, i.e., 2.8 times per km.sup.2. If a range of 50 m. is desired, the figure will be 0.45 times per km.sup.2. If incoming and outgoing traffic together are estimated at 30 mE (milliErlang) and the probability of simultaneous use of the same channel is to be a maximum of 3%, then two subscribers in the zones of the sizes mentioned can use the same duplex radio channel.
It accordingly follows that range of 20 and 50 m. a respective number of 5.6 and 0.9 subscribers per km..sup.2 and duplex radio channel can be served. These simple systems thus function well with a few radio channels as long as the subscriber density is low. However, in a city center the possibility of offering hundreds of subscribers per km..sup.2 a portable telephone must be afforded. Hundreds of duplex channels are required for this, and radio channels are a meagre resource. Frequency planning for so many radio channels is an almost impossible task at least in part because subscribers move from one place to another and take their telephones with them.